1. Field of the Invention
The present invention relates to a zoom lens and is suitable as an imaging optical system used for an image pickup apparatus such as a digital still camera, a video camera, a TV camera, or a monitoring camera.
2. Description of the Related Art
Hitherto, a zoom lens having a wide angle of field is demanded for an image pickup apparatus such as a digital camera and an image projection apparatus such as a projector. As a zoom lens having a wide angle of field, there is known a zoom lens having a wide angle of field that can perform zooming from a wide angle range having an entire imaging angle of field of 100 degrees or larger. Usually, as the zoom lens having a wide angle of field, there is widely used a negative-lead type zoom lens, which includes, in order from an object side to an image side, a first lens unit having a negative refractive power, and a rear lens group that includes one or more lens units and has a positive refractive power as a whole.
As to the negative-lead type zoom lens, there is demanded a zoom lens having a wide angle of field and the same minimum f-number (maximum aperture) over the entire zoom range so that exposure change does not occur in zooming.
A zoom lens disclosed in Japanese Patent Application Laid-Open No. 2008-046208 includes a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, a third lens unit having a negative refractive power, and a fourth lens unit having a positive refractive power. Japanese Patent Application Laid-Open No. 2008-046208 discloses a zoom lens having a wide angle of field, which has an entire imaging angle of field of 106 degrees at a wide-angle end, a zoom ratio of 2.1, and a minimum f-number of 2.8 over the entire zoom range. A zoom lens described in Japanese Patent Application Laid-Open No. 2008-233284 includes a first lens unit having a negative refractive power, and a second lens unit having a positive refractive power. Japanese Patent Application Laid-Open No. 2008-233284 discloses a zoom lens having a wide angle of field, which has an entire imaging angle of field of 113 degrees at the wide-angle end, a zoom ratio of 1.7, and a minimum f-number of 2.8 over the entire zoom range.
In the zoom lens having a wide angle of field described in Japanese Patent Application Laid-Open No. 2008-046208 or Japanese Patent Application Laid-Open No. 2008-233284, extreme peripheral light intensity of the screen is lower than or equal to 30% of that in the screen center at the wide-angle end. If a light intensity ratio in the extreme periphery of the screen is decreased in the lens having a wide angle of field in this way, a decrease of the extreme peripheral light intensity of the screen of a taken image can be recognized by eyes when a uniform luminance surface such as the sky is photographed, and the taken picture is not good.
In particular, in the zoom lens having a wide angle of field, in which the minimum f-number is constant over the entire zoom range as described in Japanese Patent Application Laid-Open No. 2008-046208 or Japanese Patent Application Laid-Open No. 2008-233284, a decrease of peripheral light intensity is apt to be conspicuous at the wide-angle end. The same is true not only for a retrofocus type optical system but also for a positive-lead type zoom lens including, in order from the object side to the image side, a first lens unit having a positive refractive power and a rear lens group including one or more lens units.